New-Tech Europe Magazine | October 2018

are not perfectly aligned in time, the transmitted signals will be misaligned and beamforming accuracy will be poor. The transmitter will be unable to maintain a stable channel from point to point. The receiver will be unable to correct for these errors. To avoid this in the time domain, these DACs need to be synchronized, meaning the data needs to be transmitted from each DAC at an identical time. Even the slightest variation in time can degrade the performance of the system. Synchronization When DACs output data at gigahertz (GHz) speeds, it is extremely difficult to synchronize their outputs across multiple devices. If a DAC is sampling at 2.7Gsps, the output code is changing every 370ps. The sample clock and the data clock from the FPGA need to be aligned for each of the transmitting DACs. The LTC2000A, a 2.7Gsps, 16- bit DAC, simplifies synchronization by including an internal register that can adjust the latency of the data through the DAC. In order to use this feature, the timing mismatch of the data lines and clock lines to each of the DACs must be within 0.4 cycles of the sample clock frequency. Figure 1 shows the ideal routing of two LTC2000As. The trace lengths of the data paths and clock paths to the individual DACs needs to match within picoseconds. This is achieved through proper digital routing techniques. If this criteria is met, then the data is guaranteed to be within one cycle from device to device. There is an internal register within the LTC2000A to program the latency of data pipeline. Each DAC register can be individually set, which will ultimately align all of the DACs in the time domain. This allows for maximum performance when all of the diversity techniques are used. Increasing the number of transmitting DACs also increases the diversity possible. The ability to synchronize many DACs

Figure 3: Typical schematic for implementing the LTC2000A

improves antenna diversity and allows for larger antenna arrays. Performance The LTC2000A also provides excellent AC performance to further improve the performance of the wireless system. Figure 2 shows the spectrum of 16 channels of CDMA with a gap channel removed. The power in each of the carriers is -36dBm and the power in the gap channel is -96dBm. This shows the excellent spectral purity of the LTC2000A. The spectral purity of the LTC2000A allows for minimal filtering of the output of the LTC2000A before transmission, simplifying the output network of the DAC. Figure 3 shows a simple network that can be used to drive an output driver from the LTC2000A. The LTC2000A updates up to 2.7Gsps, which extends the usable bandwidth beyond 1GHz. The high sample rate also provides enough bandwidth for demanding communications applications while still providing excellent spectral and noise performance. The LTC2000A’s noise spectral density is better than 158dBc/rHz for signals up to 500MHz, which keeps the signal- to-noise ratio (SNR) high for a wide range of generated frequencies. It also has a spurious free dynamic range better than 74dB up to 500MHz, and better than 65dB SFDR for output

frequencies up to 1GHz. This allows signals to be generated without spurious content that will require only minimal filtering. For the most accurate beamforming applications, the 16-bit version of the LTC2000A will provide the highest accuracy. For lower performance applications the LTC2000A has pin-compatible 14-, and 11-bit versions. The LTC2000A will improve the performance of any wireless communication system. Conclusion Modern wireless designs are constantly pushing the limits of performance. By using multiple antennas for antenna diversity, the ever present problem of multipath fading can be eliminated. The LTC2000A makes the implementation of multiple antennas possible by using the built in synchronization features. Synchronizing any number of LTC2000As can be done by simply modifying a bit in a control register. When a large number of LTC2000As are synchronized, they can be used in complex beamforming applications involving single or multiple antennas. The LTC2000A can also be used with other diversity techniques such as time, frequency and code. For more information about the LTC2000A, including design files, example layouts, signal generation programs and sample FPGA code visit www.linear.com/ LTC2000A.

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